In a final stage of printing or copying in image-forming apparatuses such as an electrophotographic copy machine, a facsimile, and a laser beam printer, a heat fixing method is generally employed in which an image is fixed by bringing a pressure roller into contact with a fixing sleeve, fixing tube, or fixing roller having a heating source therein under pressure, and causing a transfer-receiving material having a transferred toner image thereon to pass therebetween, thus melting a toner by heating.
A rubber-covered roller including a core serving as a roller base and a rubber layer for imparting elasticity, the rubber layer being disposed on the core, is widely used as a pressure roller. A known example of the rubber layer for imparting elasticity is composed of a rubber sponge, in particular, a silicone rubber sponge, containing pores (air bubbles) dispersed therein.
A silicone rubber sponge containing air bubbles dispersed therein can be produced by, for example, a method including dispersing a foaming agent in a silicone rubber, and then curing the silicone rubber while foaming the foaming agent. However, according to this method, the resulting rubber surface may become uneven because, for example, foamed cells are exposed through a wall of the silicone rubber or an irregular foaming stress is generated in the silicone rubber and the stress is released. When depressions are formed on the rubber surface, the depressions remain on the surface even in the case where a releasing layer is formed on the rubber surface, which may result in a problem of stains forming on the pressure roller due to a molten toner, etc.
To address the above problem, a method for producing a silicone rubber sponge containing air bubbles dispersed therein has also been studied in which unexpanded resin microballoons are mixed in a liquid silicone rubber and the rubber is then cured by heating while expanding the resin microballoons. However, this method tends to cause a problem of unevenness of air bubbles. Consequently, a method has also been proposed in which (expanded) resin microballoons that have been expanded in advance are mixed in a liquid silicone rubber and the silicone rubber is then cured at a temperature equal to or lower than the melting point of the resin. For example, PTL 1 discloses a silicone rubber sponge containing air bubbles (pores) formed by expanded resin microballoons dispersed therein, and a pressure roller including an elastic layer composed of this silicone rubber sponge.
This elastic layer is formed by applying, onto a core, a mixture prepared by dispersing expanded resin microballoons in a liquid silicone rubber, and curing the liquid silicone rubber by conducting heating at a temperature lower than a softening point of the resin microballoons, and the resin microballoons are broken after curing. This is because when shells of the resin microballoons remain in the resulting silicone rubber sponge, the shells may be broken as a result of thermal history during use of the silicone rubber sponge as a pressure roller and thus the fixing performance may vary. Thus, this problem is solved by breaking the shells in advance. This silicone rubber sponge is widely used as a pressure roller in an apparatus for forming a monochrome image.